Heretofore, a porous filter or membrane fabricated from an organic material and having a plurality of pores defined therein, each having a very small diameter, such as, for example, 0.05 .mu.m, has been manufactured. The film exhibits advantages such as, for example, softness and toughness. However, the film has disadvantages in that it cannot be employed within a device disposed within a high-temperature fluid due to the fact that the diameter of each pore is liable to change, the pores may be clogged with foreign material, and furthermore, a larger pressure difference is required for passing a fluid through the film. Still further the resistivity against the effects of chemical agents, particularly organic solvents is weak.
An inorganic porous separation membrane such as, for example, a porous anodic aluminum oxide film has been developed to eliminate the above-mentioned disadvantages.
In a typical anodic aluminum oxide film, each pore comprises a larger diameter portion upon an electrolyte side and a smaller diameter portion having a plurality of fine branches adjacent an aluminum metal substrate. The smaller pores are formed by reducing the applied voltage during the final step of anodic oxidation of the aluminum metal substrate (European Patent Application 178831 A1).
The porous anodic aluminum oxide film exhibits the advantages of durability, higher separation efficiency owing to large porosity, and pores being prevented from clogging because of uniformity in pore diameter. The film, however, has disadvantages in that the aluminum oxide reacts with water because of its high activity so as to produce hydrates within pores, which are accumulated within the pores, resulting in an effective reduction of the pore diameter. Consequently, the film can not be used for a long period of time for separating a fluid containing water or moisture.
It is known that the reduction of the pore diameter because of the aforenoted hydration can be prevented by denaturing the noncrystalline aluminum oxide, produced by means of the electrolysis of the aluminum substrate, into a crystalline aluminum oxide such as .gamma. alumina, by means of heat treatment at high temperatures (furnace temperatures of about 1100.degree. C.).
It is reported that an experiment in which variations of electrical properties were investigated in order to obtain data concerning the hygroscopic degree of the porous aluminum oxide film was conducted on the basis that the electrical properties are dependent upon the hygroscopic degree, and that the electrical property that is the admittance of the film, without heat treatment, was reduced to one-fifth of its initial degree within approximately 4 months which means that the hygroscopicity of the film is high, and that, to the contrary, the admittance of the film heat-treated at 1100.degree. C. did not change for one year, which means that the hygroscopicity of such film is very low.
Thus, it is understood from the facts described above that heat treatment is effective for preventing the pore diameter defined within the aluminum oxide film from changing over a period of time.
As described above, the porosity of the porous aluminum oxide is different upon its opposite sides. Accordingly, when the high temperature heat treatment of the film is performed in order to remove the hygroscopic property, the film is curved because of the difference between the porosities defined within the opposite sides. In an extreme case, the radius of its curvature can reach several millimeters. As a result, the area of the flat part of the film is severely reduced so that the film can not be used in practice.